In various fields, recently, a hard substrate surface of diamond single crystal, sapphire single crystal or the like is required to have high-level specularity and planarity.
For example, in case where diamond is used in the heat sink or the like in surface acoustic wave devices, semiconductor devices and other various devices, the diamond is required to have much more high-level specularity and planarity than before, for example, having a surface roughness in the order of nanometer or less. To satisfy the requirement, it is indispensable to develop a polishing method on a higher level for the surface of diamond single crystal.
Heretofore, for polishing the surface of diamond, mainly employed is a mechanical method. The mechanical method includes, for example, scaife polishing that comprises roughly grinding the rough surface of diamond by a physical means with a grinding machine or the like, then polishing it with diamond abrasive grains, and further melting the diamond ingredient in the rough part through high-temperature high-speed friction to thereby planarize the diamond surface.
For obtaining a desired degree of planarity in such mechanical polishing, it is necessary to control the grain size of the abrasive grains of the polishing plate; however, since the grains themselves are diamond, their grain size control is difficult. For obtaining the final planarity, the process must comprise many steps where the grain size of the abrasive grains is gradually decreased from a large one, and this is an operation requiring great skill. Nonetheless the situation could not be said to attain the necessary planarity.
Recently, sapphire single crystal is used for the substrate of blue-light emitting diodes and blue laser diodes, on which a thin film of GaN, a thin film of InGaN or the like is formed through epitaxial growth. In order that sapphire single crystal is used as the substrate for such epitaxial growth thereon, its surface must be planar, requiring a surface planarity of 2 nm or less. Recently, in place of silicon wafers, sapphire wafers resistant to radiations have become used in ultra-LSI such as devices to be mounted on satellites. Sapphire as the ultra-LSI substrate is also required to have a surface roughness in the order of 2 nm, and recently, a surface planarity more than it is required.
Heretofore in final finish of a sapphire surface, “chemical-mechanical polishing” comprising nanometer-scale mechanical removing with ultrafine abrasive grains and chemical dissolution with processed liquid having an etching effect as combined could satisfy the necessary precision level. However, with higher integration of devices, further planarization is being required, and it is being difficult to satisfy the necessary precision level with such alkali slurry.
On the other hand, the present inventors have found that, when diamond fine grains are adhered to a substrate of glass, silicon, iron, titanium, copper, plastic or the like through ultrasonic treatment and a low-temperature surface wave plasma CVD method is applied thereto, then a carbon film of carbon grains having a grain size of from 2 to 30 nm densely deposited thereon is formed to have a thickness of 2 μm or more, and that the hardness of the carbon film is 20 GPa or more (see Patent Reference 1).
Patent Reference 1: WO2005/103326